[DALI] timestamp format

[Rob Seaman]

Hi Pat,

Per 2a, see 4.4.2.2 from http://fits.gsfc.nasa.gov/standard30/fits_standard30aa.pdf (citation is A&A, v 524 (Dec 2010)

Per 2b, what does “local” otherwise mean virtually?  That is, if an ISO profile sans zed is local, that implies that its interpretation requires knowledge of the timezone (include any DST rules in effect, presumably) from some other source.  How would that be inferred / supplied in the IVOA in general and in this instance in particular?

Rob
—

On Nov 5, 2013, at 1:19 PM, Patrick Dowler <patrick.dowler at nrc-cnrc.gc.ca> wrote:

> 
> Comments from the TCG review have pointed out either an inconsistency or at least poor referencing in the PR-DALI-1.0 document.
> 
> ** The problem
> 
> The DALI document formalises the current restricted usage of ISO8601 timestamp strings within DAL services. Specifically, this common usage (I think first written down in VOResource and coming from FITS) specifies that dates are
> 
> YYYY-MM-DD
> 
> and that date+time are written as
> 
> YYYY-MM-DDThh:mm:ss[.sss]
> 
> where the T is the literal separator between date and time. This would be called a "profile" of ISO8601 since it specifies a subset of the standard. However, IVOA practice, which DALI kept, is to always require such timestamps to be UTC (detailed language is slightly more specific), while ISO8601 explicitly states that the above is interpretted as "local time" and a trailing Z is required if it is UTC.
> 
> So, common IVOA usage (and as I understand the "ISO8601 profile" in the FITS community) is not consistent with ISO8601.
> 
> ** What to do?
> 
> Well, we cannot just say this is a valid "ISO8601 profile" since stock software that groks ISO8601 will *not* interpret our timestamp strings correctly.
> 
> 1. Fix the inconsistency with ISO8601 by requiring the Z for UTC.
> 
> 2. Stick with the current style and
> 
> --2a. reference FITS usage instead of ISO8601 and *do not* state this is an ISO8601 profile, just a similar syntax
> 
> --2b. claim that in the IVOA "local" means the same as UTC (admittedly this is kind of sketchy)
> 
> --2c. is there a better justification than backwards compatibilty?
> 
> 
> If someone can provide the correct FITS reference, that would help with 2a.
> 
> Here is a good overview of ISO8601 without having to buy it:
> 
> http://www.cl.cam.ac.uk/~mgk25/iso-time.html
> 
> -- 
> 
> Patrick Dowler
> Canadian Astronomy Data Centre
> National Research Council Canada
> 5071 West Saanich Road
> Victoria, BC V9A 2L9
> 
> 250-363-0044 (office) 250-363-0045 (fax)